The present invention relates to a foamed polypropylene resin container for microwave cooking excellent in flexural rigidity under high temperature conditions.
Foamed polypropylene resin products are excellent in heat insulating ability and light weight and have been used as construction/earthmoving materials or packaging containers. Japanese Patent Application Laid-Open No. 264938/1987 discloses a multi-layered container for microwave cooking made from a foamed polypropylene resin.
Recently, foods such as curry packed in a container preserved at room temperature become popular and they are heated as packed by a microwave oven. Thus it need to be cooked for 3 minutes or longer in the case of the use of a microwave oven having a power output of 500 Watt. It results in that the temperature of the container after the heating is immediately in some cases raised up to as high as about 120 to about 140xc2x0 C. and there arises the problem that the container heated may easily be deformed.
The inventors of the present invention made intensive studies to solve the above-mentioned problem and finally have found that providing a non-foamed layer made of a polypropylene resin containing a specific amount of talc on at least one side of a foamed polypropylene resin layer prevents the resulting container from being deformed upon cooking by a microwave oven and consequently it provides an foamed polypropylene resin container for microwave cooking excellent in heat resistance.
Thus, the present invention provides a foamed polypropylene resin container for microwave cooking having a non-foamed layer containing 100 parts by weight of a polypropylene resin and 40 to 100 parts by weight of talc on at least one side of an foamed polypropylene resin layer.
The foamed polypropylene resin container for microwave cooking of the present invention is characterized in that it comprises a non-foamed layer comprising a polypropylene resin and 40 to 100 parts by weight of talc per 100 parts by weight of the polypropylene resin on at least one side of an foamed polypropylene resin layer.
The present invention relates to a foamed polypropylene resin container for microwave cooking which is excellent in rigidity under high temperature conditions, e.g., when heated in a microwave oven with curry, rice, or other foods packed therein, and therefore is hardly deformed.
The foamed polypropylene resin container is light and highly heat insulating, but its flexural rigidity is deteriorated and thus the container tends to be deformed, when the food contained therein is cooked in a microwave oven and the entire container is heated to high temperatures.
Accordingly, in the present invention, at least one side of a foamed polypropylene resin layer is provided with a non-foamed layer containing 40 to 100 parts by weight of talc per 100 parts by weight of a polypropylene resin. In view of flexural rigidity upon heating at high temperatures, the amount of the talc per 100 parts by weight of the polypropylene resin is preferably not less than 40 parts by weight. From the viewpoint of moldability of the resulting container, the amount of the talc per 100 parts by weight of the polypropylene resin is preferably not more than 100 parts by weight. For higher flexural rigidity under high temperature conditions, it is more preferred that the amount of the talc is not less than 55 parts by weight, and from the viewpoint of moldability, it is more preferred that the amount of the talc is not more than 85 parts by weight.
Moreover, incorporation of talc into the foamed layer hinders the growth of foam and results in collapsing foam during the production of a foamed polypropylene resin container for microwave cooking of the present invention or a base sheet being a constituent thereof.
Moreover, when blending talc into the foamed layer, a much larger amount of talc is required to impart the same flexural rigidity as that in the case of the non-foamed layer, leading to a large increase in the weight of the resulting container. In contrast, incorporation of talc only to the non-foamed layer makes it possible to improve flexural rigidity without making the resulting foamed polypropylene resin container for microwave cooking much heavier.
From the viewpoint of flexural rigidity, the particle size of talc dispersed in the polypropylene resin forming the non-foamed layer is preferably about 0.1 to about 10 xcexcm, more preferably about 1 to about 5 xcexcm. A foamed polypropylene resin container for microwave cooking having an higher flexural rigidity may be obtained by setting the particle size so as to fall within the range mentioned above.
In the present invention, the particle size of the talc dispersed in the polypropylene resin forming the non-foamed layer is figured out in the following manner.
A cross-section of the non-foamed layer taken in the thick direction was examined by a scanning electron microscope (SEM) at a sufficient magnification to resolve the talc particles clearly. Regarding the talc particles observable through the microscope, the maximum length of each of at least twenty talc particles was measured and the average of the values obtained was employed as the particle size of the talc dispersed in the polypropylene resin forming the non-foamed layer. Usually, the talc particles are clearly resolved at a magnification of from 5,000 to 60,000.
In the present invention, from the viewpoint of heat insulating ability of the foamed container to be obtained, the foaming ratio of the foamed layer is preferably about two times or more, more preferably about three times or more. It is preferred that, in view of the strength of the foamed container produced, the foaming ratio is about 40 times or less, more preferably about 10 times or less. The foaming ratio of the foamed layer is adjustable by controlling the amount of a foaming agent to be added or physical conditions in the molding.
Moreover, in the present invention, the term xe2x80x9cnon-foamed layerxe2x80x9d refers to a layer with a foaming ratio of from about 1.0 to about 1.5 times, preferably about 1.0 to about 1.1 times.
The thickness of the foamed polypropylene resin container for microwave cooking of the present invention is preferably from about 0.1 to about 3 mm. Containers having such thickness as to fall within the range mentioned above are preferable from the viewpoint of flexural rigidity and easy to produce.
For achieving sufficient heat insulating ability, the foamed layer is preferred to have a thickness of not less than about 0.3 mm. In view of heat insulation, the thicker the layer is, the more it is favorable.
Although the thickness of the non-foamed layer is not particularly restricted provided that its surface smoothness, in other words, external appearance is good, it is preferably about 1 xcexcm or thicker, more preferably about 10 xcexcm or thicker, with about 50 xcexcm or thicker much more preferred. The maximum limit of the thickness of the non-foamed layer is suitably set according to the intended lightweight and moldability.
In the foamed polypropylene resin container for microwave cooking of the present invention, exemplified as the polypropylene resin forming the foamed layer are a homopolymer of propylene and a propylene copolymer containing about 50 mol % or more of the propylene unit. In the propylene copolymer, preferred examples of the component copolymerizable with propylene include ethylene and xcex1-olefins having 4 to 10 carbon atoms. Included in xcex1-olefins having 4 to 10 carbon atoms are 1-butene, 4-methylpentene-1, 1-hexene, and 1-octene. As for the content of the monomer unit other than propylene, in the case of ethylene, it is preferably about 10% by weight or less. In the case of an xcex1-olefin having 4 to 10 carbon atoms, it is preferably about 30% by weight or less.
Of polypropylene resins, for their capability of forming layers with a highly homogeneous cellular structure, (a) long chain branched polypropylene resins and (b) polypropylene resins having a weight average molecular weight of 1xc3x97105 or more are prefarable.
Of these polypropylene resins, a particularly preferred one is such that, when measured by for example a Meissener-type elongation rheometer (for example, Melten rheometers manufactured by Toyo Seiki Kogyo Co., Ltd. are mentioned) at an elongation strain rate of 0.1 secxe2x88x921 and a temperature 30xc2x0 C. higher than the melting point, the ratio (xcex7100/xcex71) of its uniaxial melt elongation viscosity measured at the point where one second has passed since strain was began to be applied (xcex71) to that measured at the point where 100 seconds have passed (xcex7100) is xcex7100/xcex71xe2x89xa710.
In the present invention, the term xe2x80x9clong chain branched polypropylene resinxe2x80x9d refers to polypropylene resins having a branching degree index [A] satisfying 0.20xe2x89xa6[A]xe2x89xa60.98.
Included in examples of the long chain branched polyolefin resins having a branching degree index satisfying the formula 0.20xe2x89xa6[A]xe2x89xa60.98 is POLYPROPYLENE PF-814 manufactured by Montell.
The branching degree index is indicative of the degree of long-chain branching and is a value defined by the following expression:
Branching degree index [A]=[xcex7]Br/[xcex7]Lin 
where [xcex7]Br is the intrinsic viscosity of a long-chain branched polyolefin resin, while [xcex7]Lin is the intrinsic viscosity of a straight-chain polyolefin having the same repeating unit as the long-chain branched polyolefin resin and a weight-average molecular weight equal to that of the long-chain branched polyolefin resin.
The xe2x80x9cintrinsic viscosityxe2x80x9d also called xe2x80x9climiting viscosity numberxe2x80x9d particularly depends upon the molecular weight and branching degree of polymer molecule. Accordingly, the intrinsic viscosity serves as a measure of the branching degree of a polymer in comparing a long-chain branched polymer with a straight-chain polymer having the same weight-average molecular weight. Thus, the ratio between the foregoing intrinsic viscosities is used as the branching degree index. The method of measuring the intrinsic viscosity of polypropylene is described in Elliot et al., [J. Appl. Polym. Sci., 14, 2947-2963 (1970)]. The intrinsic viscosity of polypropylene can be measured using a sample prepared by dissolving polypropylene in tetralin or orthodichlorobenzene at 135xc2x0 C. for example. The weight-average molecular weight (Mw) of a resin can be measured by various methods, among which the method published in xe2x80x9cAmerican Laboratory, May, 63-75 (1978)xe2x80x9d by M. L. McConnel, i.e., low-angle laser light scattering intensity measuring method, is particularly preferable.
In the present invention, as an example of the way of polymerizing a polypropylene resin having a weight average molecular weight of 1xc3x97105 or more, the following method can be mentioned.
That is, in the first step, under the conditions of a polymerization temperature and a polymerization pressure, liquid propylene, triethylaluminium, t-butyl-n-propyldimethoxysilane, and a preliminary activated solid catalyst component are continuously supplied, whereby the propylene is polymerized to provide a polymer with an desired intrinsic viscosity. The polymer thus obtained is continuously sent to the second step without carrying out the deactivation of the catalyst.
In the second step, under the conditions of a polymerization temperature and a polymerization pressure, with propylene and hydrogen kept supplied so that the hydrogen concentration of the gaseous phase is constant, polymerization of the propylene is continued while the catalyst-containing polymer transferred from the first step, triethylaluminium, and t-butyl-n-propyldimethoxysilane are supplied, whereby a polymer having an desired intrinsic viscosity is provided.
The weight average molecular weight is controllable by regulating the amount of the monomer being a constituent of the resin to supply.
In addition to the polypropylene resin, the foamed layer of the foamed polypropylene resin container for microwave cooking of the present invention may contain a homopolymer of an olefin having 2 or 4 to 6 carbon atoms such as ethylene, butene, pentene, orhexene, or an olefinic copolymer constituted of two or more than two kinds of monomers having 2 or from 4 to 6 carbon atoms. The copolymer may be a block, random, or graft copolymer. The foamed layer may be made of a single olefinic copolymer, and it may be composed two or more than two kinds of olefinic copolymers. From the viewpoint of improvements in moldability, incorporation of a small amount of polyethylene is preferred.
As the foaming agent for use in the formation of the foamed layer, any foaming agent selected from chemical foaming agents and physical foaming agents is available. Usable chemical foaming agents include thermal decomposition-type foaming agents which generate nitrogen gases [e.g., azodicarbonamide, azobisisobutylonitrile, dinitrosopentamethylene tetramine, p-toluenesulfonyl hydrazide, p,pxe2x80x2-oxy-bis(benzenesulfonyl hydrazide)] and thermal decomposition-type inorganic foaming agents which generate carbon dioxide gas (e.g., sodium bicarbonate, ammonium carbonate, ammonium bicarbonate), and usable physical foaming agents include propane, butane, water, and carbon dioxide gas. Among them, substances that are inactive with respect to a high-temperature condition or fire, such as water and carbon dioxide gas, are suitable.
In the present invention, the amount of the foaming agent is not critical and, according to its type or the type of the resin, it may suitably be controlled so as to provide the desired foaming ratio.
Exemplified as the polypropylene resin forming the non-foamed layer of the foamed polypropylene resin container for microwave cooking according to the present invention are homopolymers of propylene and block, random, or graft copolymers of propylene with at least one monomer selected from olefins having 2 or 4 to 10 carbon atoms. Of these polypropylene resins, preferably employed for preventing the surface layer from getting roughened and consequently providing foamed polypropylene resin containers for microwave cooking with good external appearance are those having a melt flow rate (temperature: 230xc2x0 C., load: 2.16 kgf) (hereinafter, sometimes abbreviated to MFR) of about 5 to about 20 g/10 min., and those having a melt flow rate of about 8 to about 15 g/10 min. are particularly preferred. The melt flow rate of the polypropylene resin is measured according to JIS K7210.
The foamed polypropylene resin container for microwave cooking of the present invention may include a homopolymer of an olefin having 2 to 6 carbon atoms typified by ethylene, propylene, butene, pentene, or hexene, or a polyolefin resin such as an olefinic copolymer constituted of two or more than two kinds of monomers selected from olefins having 2 to 10 carbon atoms in its non-foamed layer. The olefin copolymer described above may be of the block, random, or graft type. The non-foamed layer may be constituted of a single kind of polyolefin resin, or it may be composed two or more than two kinds of polyolefin resins. The amount of the polyolefin resin or resins other than the polypropylene resin contained in the non-foamed layer is preferably within the range of about 1 to about 50 parts by weight per 100 parts by weight of the polypropylene resin.
The foamed polypropylene resin container for microwave cooking of the present invention may have, in addition to the above-described two essential layers, i.e., the foamed polypropylene resin layer and the non-foamed layer formed from specific amounts of a polypropylene and talc, another layer formed of a thermoplastic resin. Particularly suitable as the thermoplastic resin layer is an oriented polypropylene film (OPP), a cast polypropylene film (CPP), or a layer of an ethylene-vinyl ester copolymer saponified product.
Moreover, xe2x80x9cmodified resinsxe2x80x9d obtained by grafting, crosslinking, or modified at the molecular chain terminal are also employable.
One with a laminated structure constituted of two or more than two layers is also included in examples of those particularly preferred as the layer made from a thermoplastic resin. In the case where the thermoplastic resin layer is of the single layer type, it is preferred that its thickness is about 10 to about 100 xcexcm. If the layer is of the multilayered type, its thickness is preferred to be about 50 to about 200 xcexcm.
If the thermoplastic resin layer is of the laminated type constituted of two or more than two layers, an adhesive resin layer may optionally be provided. Exemplary of the resin for forming the adhesive resin layer is a polypropylene resin grafted onto an unsaturated carboxylic acid such as maleic acid or its anhydride.
Examples of the foamed polypropylene resin container for microwave cooking according to the present invention having two or more than two layers of thermoplastic resins laminated together are: an foamed polypropylene resin container for microwave cooking comprised of, in the order named from the interior of the container, a cast polypropylene layer, an adhesive resin layer, an ethylene-vinyl ester copolymer saponified product layer, an adhesive resin layer, a non-foamed layer, an foamed layer, and a non-foamed layer; and an foamed polypropylene resin container for microwave cooking composed of a cast polypropylene layer, an adhesive resin layer, an ethylene-vinyl ester copolymer saponified product layer, an adhesive resin layer, a non-foamed layer, an foamed layer, a non-foamed layer, a non-foamed layer, an foamed layer, and a non-foamed layer.
The foamed polypropylene resin container for microwave cooking of the present invention may have a heat seal layer. It is preferred that the heat seal layer is positioned so that it serves as the innermost layer. Preferred as the heat seal layer is one with which the lid of the container is heat-sealable and enabling the container to be sealed with the lid with a suitable adhesion strength (peeling strength). In other words, a layer whereby the lid is prevented from peeled off even under high temperature conditions of about about 120 to about 140xc2x0 C. unless force is applied intentionally but peelable by hands. An example of such heat sealing layer is a layer made of a resin composition containing 100 parts by weight of a thermoplastic resin and about 0.5 to about 160 parts by weight of finely-divided particles having a mean particle size of from about 0.05 to about 20 xcexcm selected from the group consisting of organic fine particles and inorganic fine particles. Employed as the thermoplastic resin for the heat seal layer is preferably a resin containing 100 parts by weight a polypropylene resin and about 10 to about 100 parts by weight of a polyethylene resin. A container having, in addition to two kinds of layers which are essential in the foamed polypropylene resin container for microwave cooking of the present invention, a layer of a thermoplastic resin can be produced by shaping a composite sheet made by laminating an foamed polyolefin resin sheet constituted of the above-described two essential layers (hereinafter, referred to as xe2x80x9cessential foamed sheetxe2x80x9d) with the layer of a thermoplastic resin. Examples of the method for producing the composite sheet described above include an extrusion lamination method, a sandwich lamination method according to which another thermoplastic resin is melt-extruded so as to be positioned between the thermoplastic sheet or film and the essential foamed sheet for lamination, and a method in which the thermoplastic resin sheet or film is laminated with the essential foamed sheet by melting at least one side of the essential foamed sheet through heating with hot air or an infrared heater.
Particularly preferred as the way of lamination is, from the viewpoints of lightweight of the composite foamed sheet and production cost, a heat-bonding method comprising allowing the essential foamed sheet and the layer of a thermoplastic resin to pass through a nip roll system composed of two or more rolls while blowing hot air against the nip rolls using an air knife or the like, whereby at least one side of the essential foamed sheet or the layer of a thermoplastic resin is molten and they are pressed together into a lamination by the nip rolls.
The foamed polypropylene resin container for microwave cooking of the present invention may include an additive or additives if necessary. Included in additives are antioxidants, light stabilizers, ultraviolet ray absorbers, anti-clouding agents, anti-fogging agents, plasticizers, antistatic agents, lubricants, coloring agents, dioxin inhibiting agents, ethylene gas absorbing agents, deodorants, freshness-keeping agents, and antibacterial agents.
These additives can be incorporated into either the foamed layer, or the non-foamed layer, or both.
Although there is no particular restriction as to the method of producing the foamed polypropylene resin sheet for forming the foamed polypropylene resin container for microwave cooking of the present invention, preferably employed is a method comprising extruding a molten resin from a flat die (e.g., T-die, coat hanger die), a straight die, a circular die (e.g., cross-head die) and stretching the extruded resin while foaming. Moreover, also favorable as the method of forming the foamed polypropylene resin sheet is a method in which a molten resin is extruded from a die, foamed, and stretched.
The form of the foamed polypropylene resin container for microwave cooking of the present invention is not particularly restricted and it may take the form of, for example, a tray, bowl, cup, or box.
Examples of the molding method of the foamed polypropylene resin container for microwave cooking include a process comprising softening a sheet for forming the container through heating with an infrared ray heater, shaping the softened sheet according to the vacuum, pressure, or vacuum-and-pressure molding method using a male mold, a female mold, or a combination of these, and cooling the resulting product for solidification; and a process which does not utilize the vacuum, pressure, or vacuum-and-pressure molding technique, according to which a sheet for forming the foamed polypropylene resin container for microwave cooking of the present invention is supplied between two molds which fit with each other and pressed into the desired form.
In the molding method described above, before vacuum- or press-shaping the sheet with either a female mold or a male mold, or after the molding, a plug having a shape similar to that of the container may be brought into contact with the sheet thereby to shape into the desired form preliminary.
An example of the process for providing the foamed polypropylene resin container for microwave cooking of the present invention comprises the following steps.
(1) A heating step wherein the periphery of a base sheet from which the foamed polypropylene resin container for microwave cooking of the present invention is formed is held/fixed with clips and softened through heating.
(2) A molding step wherein a female mold having such a shape as to correspond to the external shape of the foamed polypropylene resin container for microwave cooking of the present invention and a plug the shape of which is in conformity with the internal shape of the container are positioned so as to be opposite to each other in relation to the base sheet; the softened base sheet is pre-shaped by bringing the plug into abutting relation with the base sheet and relatively moving in the direction of the female mold; the female mold is brought into contact with the base sheet; and the pressure between the female mold and the base sheet is reduced to establish tight contact between the surface of the female mold and the base sheet thereby to mold the sheet in the form of the foamed polypropylene resin container for microwave cooking of the present invention.
(3) A mold-releasing step wherein the article molded in the form of the foamed polypropylene resin container for microwave cooking of the present invention is detached from the mold.
The foamed polypropylene resin container for microwave cooking of the present invention can be obtained according to the molding method described above. The use of a plurality of female molds makes it possible to mold many containers at one time through a single sequence of steps.
It is preferred that the above-described process incorporates, following the heat-softening step (1), a swelling step wherein a difference in pressure is developed across the heat-softened base sheet to allow the sheet to bulge in the direction of the lower-pressure side, and that some changes in the molding method are made so that, in the molding step (2), the plug is positioned on the lower-pressure side of the base sheet and allowed to abut against the softened base sheet from the direction in which the sheet bulges. This enables the entire sheet to be stretched evenly before being shaped with the molds, preventing local unevenness in stretching, which consequently makes it possible to provide containers having a high drawn ratio as well as containers excellent in rigidity.
The drawn ratio as designated herein is a value defined by the ratio of the height of the container to the minimum dimension of the opening of the container. The larger the drawn ratio is, the thinner the sidewall of the container tends to become. In addition, the rigidity of the resulting container is liable to get deteriorated. As employed herein minimum dimension of the opening of the container refers to, if the shape of the opening of the container is circular, the inside diameter of the opening. If the shape of the opening is square, it refers to the length of a side. If the shape of the opening is rectangular, it refers to the length of a shorter side. In the case where the opening of the container is hexagonal in shape, it refers to the distance between the opposing sides.
The high temperature flexural rigidity of the foamed polypropylene resin container for microwave cooking of the present invention is evaluated by observing the degree of deformation of the container when heated in a microwave oven with something suitable packed therein.
For example, the rigidity of the container under high temperature conditions can be evaluated by: fabricating an foamed polypropylene resin container for microwave cooking having an opening diameter of 130 mm, a periphery width of 10 mm, a bottom diameter of 60 mm, and a height of 50 mm; filling the container with 230 g of curry; and observing the degree of deformation of the container when heated in a 500 Watt microwave oven for 4 minutes.
Moreover, the high temperature flexural rigidity of the foamed polypropylene resin container for microwave cooking of the present invention can also be evaluated by measuring the modulus of bending elasticity of the materials of the container at high temperatures.
For example, the high temperature flexural rigidity can be evaluated by laterally cutting a specimen which is 20 mm in width and 80 mm in length from the side wall of the foamed polypropylene resin container for microwave cooking of the present invention and measuring the modulus of bending elasticity at 120xc2x0 C. using an autograph. For example, a bending elasticity of 800 kg/cm2 or more shows that the container is excellent in flexural rigidity at high temperatures, and a bending elasticity of less than 800 kg/cm2 indicates that the container is poor in flexural rigidity under high temperature conditions.